This invention relates to a method of controlling the operation of a fan or compressor to avoid surge and/or stall and/or flutter, and is particularly, although not exclusively, concerned with fans or compressors for aircraft use. Such fans or compressors are typically driven by a turbine stage of a gas turbine engine by means of a driveshaft.
For convenience, the expression “compressor” is used in this specification to embrace fans, which discharge gas (usually air) directly into the surroundings to provide a propulsive force, or discharged into a pipe/duct so as to be pumped along the pipe/duct, and compressors which compress a working fluid (again, usually air) which is subsequently mixed with fuel and ignited either to provide a propulsive jet flow or to drive a turbine, or a combination of the two.
In steady state operation, a compressor operates on a working line, determined by the effective exit area of the compressor, in a stable manner. However, under some operating conditions, particularly when the compressor is operating at high speed and supporting a high pressure ratio, the compressor operation may be such that at some operating points the working line can approach a stability line. If the compressor operates beyond the stability line, stall or surge may occur, which result from the breakdown of the air flow through the compressor. Surge can have a major effect on compressor output, leading to a loss of thrust, and possibly damage to the compressor itself or other parts of the engine from which it is driven. Another influence on compressor stability is known as flutter, which is a self-excited oscillation that occurs in compressor aerofoils. This can result in fatigue damage and/or failure of the aerofoils. A flutter stability line can be defined in a similar way to that for surge/stall.
It is consequently important to monitor the operating point of the compressor and to control its operation to avoid surge or other detrimental instabilities. U.S. Pat. No. 7,094,019 discloses one such monitoring method, in which the current pressure ratio across the compressor is compared with the compressor's pressure ratio at surge limit conditions. A predetermined safety margin is established, expressed as a percentage of the total span of the compressor's pressure ratio.
It is known for such safety margins to be defined by identifying marginal “pinch points” in the operational range of the compressor and to build in threats at these pinch points and then to prove empirically that, on a statistical basis, the likelihood of all of the threats arising at the same time at a pinch point is vanishingly small, so that the safety margin is adequate and the compressor is safe from surge and other related instabilities. By “pinch points” is meant regions of the stability line which approach the working line of the compressor more closely that other regions.
Where intakes to the compressor have little or no variable geometry, the working line of the compressor is established by fixing the geometric nozzle area so that it is sufficiently far from the stability line to prevent surge at the most demanding operational condition.
However, some engines or propulsive fans have multiple variable geometries. For example, the compressor may have variable inlet guide vanes that can vary independently of fan speed, a variable nozzle area, variable nozzle geometry (and hence discharge coefficient). Adjustment of any of these features will vary the working line of the compressor. Additionally, inlet flow distortion can be a severe threat to surge margin, the effect of such distortion varying with flight condition for some intake configurations.
If a fixed safety margin is established to accommodate all of these threats under different flight conditions, the result is a working line which is so far from the stability line as to be impractical. Thus, although a fixed safety margin can be established which results in a nozzle area that prevents surge at the worst condition, the safety margin will prohibit the compressor from producing sufficient thrust, or operating at maximum efficiency, at other flight conditions.